1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device using a crystalline thin-film semiconductor and, more particularly, to a method of fabricating thin-film transistors (TFTs).
2. Description of the Related Art
In recent years, transistors (known as TFTs) using a thin-film semiconductor formed on a substrate made of glass or quartz have attracted attention. In particular, a thin-film semiconductor having a thickness of hundreds of thousands of angstroms is formed on the surface of the substrate of glass or quartz. Transistors (gate-insulated field-effect transistors) are formed, using this thin-film semiconductor.
It is known that these thin-film transistors can be applied to active matrix liquid crystal displays. In such an active matrix liquid crystal display, hundreds of thousands of pixels or more pixels are arranged in rows and columns. TFTs are arranged as switching devices for these pixels, respectively, to provide a high-speed display with high information content.
Of these TFTs used in such active matrix liquid crystal displays, TFTs using amorphous thin silicon films have been put into practical use. However, these TFTs using amorphous thin silicon films have the problem that their characteristics are poor. For example, where an active matrix liquid crystal display having higher display performance should be constructed, the characteristics of the TFTs using amorphous thin silicon films are not satisfactory.
Furthermore, an integrated liquid crystal display system has been proposed. In this system, the peripheral driver circuit is made up of TFTs similarly to switching devices for pixels. All of these TFTs are packed on a single substrate. Unfortunately, TFTs using amorphous thin silicon films cannot construct the peripheral driver circuit because of their low operating speed. Especially, with TFTs using amorphous thin silicon films, it is difficult to fabricate a practical P-channel type. That is, the characteristics are so poor that any practical device cannot be obtained. Hence, there is the fundamental problem that CMOS circuits cannot be built.
In addition, a technique for forming an integrated circuit for processing or storing image data or the like integrally with a pixel region and a peripheral driver circuit on the same substrate has been proposed. However, with TFTs using amorphous thin silicon films, an integrated circuit capable of processing image data cannot be constructed because of their poor characteristics.
A further technique for fabricating a higher-performance TFT from a crystalline silicon film is known. The characteristics of this higher-performance TFT is much better than those of a TFT using an amorphous thin silicon film. In this known technique, after formation of the amorphous silicon film, a heat treatment or laser irradiation is performed to change the amorphous silicon film into a crystalline silicon film. This crystalline silicon film obtained by crystallizing the amorphous silicon film generally has a polycrystalline or crystallitic structure.
Where a TFT is fabricated, using a crystalline silicon film, characteristics much better than those obtained when an amorphous silicon film is used can be obtained. The mobility of a TFT is one index used to evaluate its characteristics. An N-channel TFT using an amorphous silicon film has a mobility of 1 to 2 cm.sup.2 /Vs or less. An N-channel TFT using a crystalline silicon film has a mobility of more than about 100 cm.sup.2 /Vs. A P-channel TFT using a crystalline silicon film has a mobility of more than about 50 cm.sup.2 /Vs.
However, a crystalline silicon film obtained by crystallizing an amorphous silicon film has a polycrystalline structure and many problems due to crystal grain boundaries exist. For example, some carriers move through the grain boundaries. This severely limits the withstand voltage of the TFT. Furthermore, when it is operated at a high speed, a variation or deterioration in characteristics tends to occur. Additionally, carriers moving through grain boundaries increase the OFF current, or leakage current.
Where one attempts to construct an integrated active matrix liquid crystal display having a higher device density, it is desired to form a peripheral circuit on one glass substrate, along with a pixel region. In this case, in order to drive hundreds of thousands of pixel transistors arranged in rows and columns, TFTs arranged in the peripheral circuit are required to handle large currents.
In order to obtain TFTs capable of treating large currents, it is necessary to adopt a structure having an increased channel width. However, if the channel width of TFTs using a crystalline silicon film is increased, the resulting withstand voltage makes it impossible to use the TFTs in practice. Furthermore, the threshold value varies so greatly that the TFTs cannot be put into practical use.
When one tries to construct an integrated circuit for processing image data from TFTs using crystalline silicon films, it has been impossible to obtain practical integrated circuit (capable of replacing the prior art IC) because of variations in threshold value and by aging effects.